Theory of Operation
Blood Pump Flow Path
The EVAHEART Blood Pump is designed to assist the native heart by removing blood volume from the patient’s left ventricle and pumping it into the aorta, thereby increasing blood flow while reducing the workload on the left ventricle. Blood travels from the left ventricle, through the Inflow Cannula, and into the Blood Pump, where a rotating impeller pushes it toward the outer edge of the pump chamber, through the volute, and out of the Blood Pump outlet. From the Blood Pump outlet, blood flows through the Outflow Graft, and into the ascending aorta.
Rotational Speed and Head Pressure
The EVAHEART Blood Pump is a centrifugal pump with an open, swept-back impeller design. Blood enters the pump chamber through the blood inlet just above the rotating impeller. As the fluid passes over the impeller, it imparts kinetic energy to the fluid until it clears the edge of the impeller. Passing through the impeller increases both fluid velocity and fluid pressure. Blood exits the blood pump chamber and moves through the volute where it slows down, converting a large part of the kinetic energy into additional fluid pressure.1
Blood flow through the pump is a function of impeller speed and the pressure differential between the pump inlet and pump outlet (or head pressure). This performance is characterized by a pressure versus flow or “HQ” curve, shown below. At a fixed pump speed, as the pressure differential across the pump (Δp) increases, the flow generated by the pump decreases. Likewise, as the pressure differential across the pump (Δp) decreases, the flow generated by the pump increases.
Left Ventricular Pressure and Aortic Pressure
During the left ventricle’s systolic phase, contraction of the left ventricle increases blood pressure at the pump’s inlet, causing the pressure differential across the pump to decrease. As a result, flow through the pump increases. Blood flows into the vascular system and meets vascular resistance, increasing pressure at the pump outlet. In the diastolic phase, the left ventricle relaxes, reducing the LVP but increasing the pressure differential across the pump. Referring to the HQ curve below, with pump speed constant, as the pressure differential (Δp) increases, flow decreases. This flow remains low until the left ventricle contracts again to start another cardiac cycle. As a result, flow generated by the Blood Pump is pulsatile and in synchrony with contractions of the left ventricle. The amplitude of pulsation depends upon the pump speed, aortic pressure, and the contractility of the native left ventricle.
Blood Pump Performance
The following curve demonstrates the pressure and flow capabilities of the EVAHEART Blood Pump. Based on this curve it is possible to select an impeller speed for a known pressure differential to generate a desired flow. For example, when the impeller is rotating at 2,000 revolutions per minute (rpm), the system can produce a flow rate of up to 15 liters per minute (L/min) with a pressure head of approximately 35 mmHg.
1 Frank White, Fluid Mechanics (New York, NY: McGraw-Hill, 1979), p. 636